An atoll ( // , // , // , // , // or // ), sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. (p60) The coral of the atoll often sits atop the rim of an extinct seamount or volcano which has eroded or subsided partially beneath the water. The lagoon forms over the volcanic crater or caldera while the higher rim remains above water or at shallow depths that permit the coral to grow and form the reefs. For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upward and outward to replace the lost height.
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world.
A coral island is a type of island formed from coral detritus and associated organic material. They occur in tropical and sub-tropical areas, typically as part of coral reefs which have grown to cover a far larger area under the sea.
The word atoll comes from the Dhivehi (an Indo-Aryan language spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު, [ˈət̪ɔɭu] ), meaning an administrative subdivision. OED Its first recorded use in English was in 1625 as atollon. Charles Darwin recognized its indigenous origin and coined, in his The Structure and Distribution of Coral Reefs , the definition of atolls as "circular groups of coral islets" that is synonymous with "lagoon-island". (p2)
The Maldives, officially the Republic of Maldives, are an Asian country, located in the Indian Ocean, situated in the Arabian Sea. The country lies southwest of Sri Lanka and India, about 1,000 kilometres (620 mi) from the Asian continent. The chain of 26 atolls stretches from Ihavandhippolhu Atoll in the north to the Addu City in the south. Comprising a territory spanning roughly 298 square kilometres (115 sq mi), the Maldives is one of the world's most geographically dispersed sovereign states as well as the smallest Asian country by land area and population, with around 427,756 inhabitants. Malé is the capital and a populated city, traditionally called the "King's Island" for its central location.
Charles Robert Darwin, was an English naturalist, geologist and biologist, best known for his contributions to the science of evolution. His proposition that all species of life have descended over time from common ancestors is now widely accepted, and considered a foundational concept in science. In a joint publication with Alfred Russel Wallace, he introduced his scientific theory that this branching pattern of evolution resulted from a process that he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding.
More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus"or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".
A promontory is a raised mass of land that projects into a lowland or a body of water.
An islet is a very small island.
Most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Lakshadweep Islands, the Chagos Archipelago and the Outer Islands of the Seychelles). The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean.
The Caroline Islands are a widely scattered archipelago of tiny islands in the western Pacific Ocean, to the north of New Guinea. Politically they are divided between the Federated States of Micronesia in the eastern part of the group, and Palau at the extreme western end. Historically, this area was also called Nuevas Filipinas or New Philippines as they were part of the Spanish East Indies and governed from Manila in the Philippines.
The Marshall Islands, officially the Republic of the Marshall Islands, is an island country and a United States associated state near the equator in the Pacific Ocean, slightly west of the International Date Line. Geographically, the country is part of the larger island group of Micronesia. The country's population of 53,158 people is spread out over 29 coral atolls, comprising 1,156 individual islands and islets.
The Coral Sea Islands Territory is an external territory of Australia which comprises a group of small and mostly uninhabited tropical islands and reefs in the Coral Sea, northeast of Queensland, Australia. The only inhabited island is Willis Island. The territory covers 780,000 km2 (301,160 sq mi), most of which is ocean, extending east and south from the outer edge of the Great Barrier Reef and includes Heralds Beacon Island, Osprey Reef, the Willis Group and fifteen other reef/island groups. Cato Island is the highest point in the Territory.
Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are only found in the tropics and subtropics. The northernmost atoll of the world is Kure Atoll at 28°24′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls of the world are Elizabeth Reef at 29°58′ S, and nearby Middleton Reef at 29°29′ S, in the Tasman Sea, both of which are part of the Coral Sea Islands Territory. The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°40′ S. Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°24′ N. At this latitude coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different mode of formation. While there is no atoll directly on the equator, the closest atoll to the Equator is Aranuka of Kiribati, with its southern tip just 12 km north of the equator.
The tropics are the region of the Earth surrounding the Equator. They are delimited in latitude by The Tropic of Cancer in the Northern Hemisphere at 23°26′12.4″ (or 23.43678°) N and the Tropic of Capricorn in the Southern Hemisphere at 23°26′12.4″ (or 23.43678°) S; these latitudes correspond to the axial tilt of the Earth. The tropics are also referred to as the tropical zone and the torrid zone. The tropics include all the areas on the Earth where the Sun contacts a point directly overhead at least once during the solar year - thus the latitude of the tropics is roughly equal to the angle of the Earth's axial tilt.
The subtropics are geographic and climate zones located roughly between the tropics at latitude 23.5° and temperate zones north and south of the Equator.
Kure Atoll or Ocean Island is an atoll in the Pacific Ocean 48 nautical miles beyond Midway Atoll in the Northwestern Hawaiian Islands at. The only land of significant size is called Green Island and is a habitat for hundreds of thousands of seabirds. A short, unused and unmaintained runway and a portion of one building, both from a former United States Coast Guard LORAN station, are located on the island. Politically it is part of Hawaii, although separated from the rest of the state by Midway, which is a separate unorganized territory. Green Island, in addition to being the nesting grounds of tens of thousands of seabirds, has recorded several vagrant terrestrial birds including snow bunting, eyebrowed thrush, brambling, olive-backed pipit, black kite, Steller's sea eagle and Chinese sparrowhawk.
|Great Chagos Bank||Land area 4.5 km2|
|Reed Bank||Submerged, at shallowest 9 m|
|Macclesfield Bank||Submerged, at shallowest 9.2 m|
|North Bank (Ritchie Bank, north of|
Saya de Malha Bank)
|Submerged, at shallowest <10 m|
|Rosalind Bank||Submerged, at shallowest 7.3 m|
|Two names, but a single atoll structure;|
land area 51 km2
|Chesterfield Islands||Land area <10 km2|
|Huvadhu Atoll||Land area 38.5 km2|
|Nukuoro atoll||Land area 1.7 km2, divided among more|
than 40 islets that lie on the northern, eastern
and southern sides of the lagoon
|Lihou Reef||Land area 1 km2|
|Bassas de Pedro||Submerged, at shallowest 16.4 m|
|Ardasier Bank||Cay on the south side?|
|Kwajalein||Land area 16.4 km2|
|Diamond Islets Bank||Land area <1 km2|
|Namonuito Atoll||Land area 4.4 km2|
|Ari Atoll||Land area 69 km2|
|Rangiroa||Land area 79 km2|
|Kolhumadulhu Atoll||Land area 79 km2|
|Kaafu Atoll (North Malé Atoll)||Land area 69 km2|
|Ontong Java||Land area 12 km2|
In most cases, the land area of an atoll is very small in comparison to the total area. Atoll islands are low lying, with their elevations less than 5 meters (9). Measured by total area, Lifou (1146 km2) is the largest raised coral atoll of the world, followed by Rennell Island (660 km2). More sources however list as the largest atoll in the world in terms of land area Kiritimati, which is also a raised coral atoll (321.37 km2 land area; according to other sources even 575 km2), 160 km2 main lagoon, 168 km2 other lagoons (according to other sources 319 km2 total lagoon size). The remains of an ancient atoll as a hill in a limestone area is called a reef knoll. The second largest atoll by dry land area is Aldabra with 155 km2. The largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives with 255 islands.
Lifou[lifu] is a commune of France in the Loyalty Islands Province of New Caledonia in the Pacific Ocean.
A raised coral atoll or uplifted coral atoll is an atoll that has been lifted high enough above sea level by tectonic forces to protect it from scouring by storms and enable soils and diverse – often endemic – species of flora and fauna to develop. With the exception of Aldabra Island in the Indian Ocean and Henderson Island in the Pacific, most tropical raised atolls have been dramatically altered by human activities such as species introduction, phosphate mining and even bomb testing.
Rennell Island, locally known as Mugaba, is the main island of two inhabited islands that make up the Rennell and Bellona Province in the Solomon Islands. Rennell Island has a land area of 660 square kilometres (250 sq mi) that is about 80 kilometres (50 mi) long and 14 kilometres (8.7 mi) wide. It is the second largest raised coral atoll in the world with the largest lake in the insular Pacific, Lake Tegano, a lake that is listed as a World Heritage Site. Rennell Island has a population of about 1,840 persons of Polynesian descent who primarily speak Rennellese, Pijin and some English. Rennell and Bellona Islands are two of the few islands in the Melanesian Solomon Island archipelago classified as a Polynesian outlier; others being Sikaiana, Ontong Java, Tikopia, Anuta, Duff Islands, and some Reef Islands.
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In 1842, Charles Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard HMS Beagle from 1831 to 1836. Accepted as basically correct, his explanation involved considering that several tropical island types—from high volcanic island, through barrier reef island, to atoll—represented a sequence of gradual subsidence of what started as an oceanic volcano. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upward as the island subsides (sinks), becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, Bora Bora and others in the Society Islands. The fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the coral and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface and the barrier reef remains. At this point, the island has become an atoll.
Atolls are the product of the growth of tropical marine organisms, and so these islands are only found in warm tropical waters. Volcanic islands located beyond the warm water temperature requirements of hermatypic (reef-building) organisms become seamounts as they subside and are eroded away at the surface. An island that is located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point. Islands in colder, more polar regions evolve toward seamounts or guyots; warmer, more equatorial islands evolve toward atolls, for example Kure Atoll.
Reginald Aldworth Daly offered a somewhat different explanation for atoll formation: islands worn away by erosion, by ocean waves and streams, during the last glacial stand of the sea of some 900 feet (270 m) below present sea level developed as coral islands (atolls), or barrier reefs on a platform surrounding a volcanic island not completely worn away, as sea level gradually rose from melting of the glaciers. Discovery of the great depth of the volcanic remnant beneath many atolls such as at Midway Atoll favors the Darwin explanation, although there can be little doubt that fluctuating sea level has had considerable influence on atolls and other reefs.
Coral atolls are also an important place where dolomitization of calcite occurs. At certain depths water is undersaturated in calcium carbonate but saturated in dolomite. Convection created by tides and sea currents enhance this change. Hydrothermal currents created by volcanoes under the atoll may also play an important role.
In 1896, 1897 and 1898, the Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls. This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin in the Pacific.
The first expedition in 1896 was led by Professor William Johnson Sollas of the University of Oxford. The geologists included Walter George Woolnough and Edgeworth David of the University of Sydney. Professor Edgeworth David led the expedition in 1897.The third expedition in 1898 was led by Alfred Edmund Finckh.
On January 6, 2009, U.S. President George W. Bush announced the creation of the Pacific Remote Islands Marine National Monument, covering several islands and atolls under U.S. jurisdiction. Number 1, page 14)(
The Western Pacific nation of Tuvalu, formerly known as the Ellice Islands, is situated 4,000 kilometers (2,500 mi) and is 26 km northeast of Australia and is approximately halfway between Hawaii and Australia. It lies east-northeast of the Santa Cruz Islands, southeast of Nauru, south of Kiribati, west of Tokelau, northwest of Samoa and Wallis and Futuna and north of Fiji.
In marine geology, a guyot, also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than 200 m (660 ft) below the surface of the sea. The diameters of these flat summits can exceed 10 km (6.2 mi). Guyots are most commonly found in the Pacific Ocean, but they have been identified in all the oceans except the Arctic Ocean.
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface, and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to 1,000–4,000 m (3,300–13,100 ft) in height. They are defined by oceanographers as independent features that rise to at least 1,000 m (3,281 ft) above the seafloor, characteristically of conical form. The peaks are often found hundreds to thousands of meters below the surface, and are therefore considered to be within the deep sea. During their evolution over geologic time, the largest seamounts may reach the sea surface where wave action erodes the summit to form a flat surface. After they have subsided and sunk below the sea surface such flat-top seamounts are called "guyots" or "tablemounts"
The Northwestern Hawaiian Islands or the Leeward Islands are the small islands and atolls in the Hawaiian island chain located northwest of the islands of Kauai and Niihau. Politically, they are all part of Honolulu County in the U.S. state of Hawaii, except Midway Atoll, which is a territory distinct from Hawaii and grouped as one of the United States Minor Outlying Islands. The United States Census Bureau defines this area, except Midway, as Census Tract 114.98 of Honolulu County. Its total land area is 3.1075 square miles (8.048 km2). All the islands except Nihoa are north of the Tropic of Cancer, making them the only islands in Hawaii that lie outside the tropics.
The fifteen volcanoes that make up the eight principal islands of Hawaii are the youngest in a chain of more than 129 volcanoes that stretch 5,800 kilometres (3,600 mi) across the North Pacific Ocean, called the Hawaiian-Emperor seamount chain. Hawaiʻi's volcanoes rise an average of 4,572 metres (15,000 ft) to reach sea level from their base. The largest, Mauna Loa, is 4,169 metres (13,678 ft) high. As shield volcanoes, they are built by accumulated lava flows, growing a few meters/feet at a time to form a broad and gently sloping shape.
The Egan Islands or Egan Atoll, also known as Six Iles, is an uninhabited atoll administered by the United Kingdom. They are one of the few emerged coral atolls that make up the Chagos Archipelago, British Indian Ocean Territory.
In geology, a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs.
A fringing reef is one of the four main types of coral reef recognized by most coral reef scientists. It is distinguished from the other main types in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows directly from the shoreline the reef flat extends right to the beach and there is no backreef. In other cases, fringing reefs may grow hundreds of yards from shore and contain extensive backreef areas with numerous seagrass meadows and patch reefs.
The Structure and Distribution of Coral Reefs, Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836, was published in 1842 as Charles Darwin's first monograph, and set out his theory of the formation of coral reefs and atolls. He conceived of the idea during the voyage of the Beagle while still in South America, before he had seen a coral island, and wrote it out as HMS Beagle crossed the Pacific Ocean, completing his draft by November 1835. At the time there was great scientific interest in the way that coral reefs formed, and Captain Robert FitzRoy's orders from the Admiralty included the investigation of an atoll as an important scientific aim of the voyage. FitzRoy chose to survey the Keeling Islands in the Indian Ocean. The results supported Darwin's theory that the various types of coral reefs and atolls could be explained by uplift and subsidence of vast areas of the Earth's crust under the oceans.
Nintoku Seamount or Nintoku Guyot is a seamount and guyot in the Hawaiian-Emperor seamount chain. It is a large, irregularly shaped volcano that last erupted 66 million years ago. Three lava flows have been sampled at Nintoku Seamount; the flows are almost all alkalic (subaerial) lava. It is 56.2 million years old.
The Mid-Pacific Mountains (MPM) is a large oceanic plateau located in the central North Pacific Ocean or south of the Hawaiian–Emperor seamount chain. Of volcanic origin and Mesozoic in age, it is located on the oldest part of the Pacific Plate and rises up to 2 km (1.2 mi) above the surrounding ocean floor and is covered with several layers of thick sedimentary sequences that differ from those of other plateaux in the North Pacific. About 50 seamounts are distributed over the MPM. Some of the highest points in the range are above sea level which include Wake Island and Marcus Island.
Wōdejebato is a Cretaceous guyot or tablemount in the northern Marshall Islands, Pacific Ocean. Wōdejebato is probably a shield volcano and is connected through a submarine ridge to the smaller Pikinni Atoll 74 kilometres (46 mi) southeast of the guyot; unlike Wōdejebato, Pikinni rises above sea level. The seamount rises for 4,420 metres (14,500 ft) to 1,335 metres (4,380 ft) depth and is formed by basaltic rocks. The name Wōdejebato refers to a sea god of Pikinni.
Limalok is a Cretaceous-Paleocene guyot/tablemount in the southeastern Marshall Islands, one of a number of seamounts in the Pacific Ocean. It was probably formed by a volcanic hotspot in present-day French Polynesia. Limalok lies southeast of Mili Atoll and Knox Atoll, which rise above sea level, and is joined to each of them through a volcanic ridge. It is located at a depth of 1,255 metres (4,117 ft) and has a summit platform with an area of 636 square kilometres (246 sq mi).
Lo-En or Hess is an Albian-Campanian guyot in the Marshall Islands. One among a number of seamounts in the Pacific Ocean, it was probably formed by a hotspot in what is present-day French Polynesia. Limalok lies southeast of Eniwetok which rise above sea level and Lo-En is almost connected to it through a ridge.
Allison Guyot is a tablemount (guyot) in the underwater Mid-Pacific Mountains of the Pacific Ocean. It is a trapezoidal flat mountain rising 1,500 metres above the seafloor, with a summit platform 35 by 70 kilometres wide. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of their formation were located in the Southern Hemisphere.
Darwin Guyot is a volcanic underwater mountain top, or guyot, in the Mid-Pacific Mountains between the Marshall Islands and Hawaii. Named after Charles Darwin, it rose above sea level more than 118 million years ago during the early Cretaceous period to become an atoll, developed rudist reefs, and then drowned, perhaps as a consequence of sea level rise. The flat top of Darwin Guyot now rests 1,266 metres (4,154 ft) below sea level.
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